Fix torch.intx support in FakeQuantizeConfig (#1544)

**Summary:** Fixes the following error when passing `torch.intx` to
`FakeQuantizeConfig`. These dtypes were introduced in PyTorch 2.6+:

```
ValueError: Unsupported dtype 'torch.int4', choose from [torch.int8, torch.uint8, <TorchAODType.INT1: 1>, <TorchAODType.INT2: 2>, <TorchAODType.INT3: 3>, <TorchAODType.INT4: 4>, <TorchAODType.INT5: 5>, <TorchAODType.INT6: 6>, <TorchAODType.INT7: 7>, torch.uint1, torch.uint2, torch.uint3, torch.uint4, torch.uint5, torch.uint6, torch.uint7]
```

**Test Plan:**
python test/quantization/test_qat.py -k test_fake_quantize_config_torch_intx

Author: andrewor14

test/quantization/test_qat.py

@@ -63,6 +63,7 @@
 from torchao.utils import (
     TORCH_VERSION_AT_LEAST_2_3,
     TORCH_VERSION_AT_LEAST_2_4,
+    TORCH_VERSION_AT_LEAST_2_6,
 )
 
 # TODO: put this in a common test utils file
@@ -1327,6 +1328,26 @@ def test_quantize_api_convert_path(self):
         baseline_out = baseline_model(*x2)
         torch.testing.assert_close(out, baseline_out, atol=0, rtol=0)
 
+    @unittest.skipIf(
+        not TORCH_VERSION_AT_LEAST_2_6, "skipping when torch version is 2.6 or lower"
+    )
+    def test_fake_quantize_config_torch_intx(self):
+        """
+        Test that `FakeQuantizeConfig` works with torch.intx.
+        """
+        group_size = 16
+        config1 = FakeQuantizeConfig(TorchAODType.INT4, group_size=group_size)
+        config2 = FakeQuantizeConfig(torch.int4, group_size=group_size)
+        linear1 = FakeQuantizedLinear(32, 64, weight_config=config1)
+        linear2 = FakeQuantizedLinear(32, 64, weight_config=config2)
+        linear2.weight = linear1.weight
+        torch.manual_seed(self.SEED)
+        x = torch.randn((1, 32)).to(torch.float)
+        x2 = copy.deepcopy(x)
+        out1 = linear1(*x)
+        out2 = linear2(*x2)
+        torch.testing.assert_close(out1, out2, atol=0, rtol=0)
+
 
 if __name__ == "__main__":
     unittest.main()

torchao/quantization/quant_primitives.py

@@ -18,6 +18,7 @@
 from torchao.utils import (
     TORCH_VERSION_AT_LEAST_2_3,
     TORCH_VERSION_AT_LEAST_2_5,
+    TORCH_VERSION_AT_LEAST_2_6,
     _is_float8_type,
     _register_custom_op,
 )
@@ -162,6 +163,31 @@ class TorchAODType(Enum):
         }
     )
 
+# torch.intX available only in PyTorch 2.6+
+if TORCH_VERSION_AT_LEAST_2_6:
+    _SUB_BYTE_INT_BOUNDS.update(
+        {
+            torch.int1: (-(2**0), 2**0 - 1),
+            torch.int2: (-(2**1), 2**1 - 1),
+            torch.int3: (-(2**2), 2**2 - 1),
+            torch.int4: (-(2**3), 2**3 - 1),
+            torch.int5: (-(2**4), 2**4 - 1),
+            torch.int6: (-(2**5), 2**5 - 1),
+            torch.int7: (-(2**6), 2**6 - 1),
+        }
+    )
+    _DTYPE_TO_BIT_WIDTH.update(
+        {
+            torch.int1: 1,
+            torch.int2: 2,
+            torch.int3: 3,
+            torch.int4: 4,
+            torch.int5: 5,
+            torch.int6: 6,
+            torch.int7: 7,
+        }
+    )
+
 _DTYPE_TO_QVALUE_BOUNDS.update(_SUB_BYTE_UINT_BOUNDS)
 _DTYPE_TO_QVALUE_BOUNDS.update(_SUB_BYTE_INT_BOUNDS)
 assert _DTYPE_TO_BIT_WIDTH.keys() == _DTYPE_TO_QVALUE_BOUNDS.keys()